How about one over each fender?
A small tray behind the fender (mounted to the frame and/or rear bumper) for the bike's rear wheel, and "fork mount" mounted linear on the fender to allow the bars to be inline with the bike frame. If the crankset is too wide, buy a couple of folding pedals or remove the inside pedal for transport!
This design would be solid and not add width to the trailer. the front wheels could be placed in a bag in the TV, the trailer, or even bungeed to the bike frame.
It would also be very easy to put the bikes on and off each rack.
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Another thought...
Dahon folding bikes are about half the price of an entry level decent bike. Maybe racks designed to mount on each end of the bumper. One for each folded bike, leaving the door exposed for entry.
I can see either of these choices being solid, simple and easy to engineer. The bumper mount might require adding another set of
tail lights, but then high mount LED taillights would be a big plus anyway.